$4.($Exercise; Section $11\mathrm{.}4)$Someone sitting in the stands of a $($perfectly frictionless$)$ ice rink sees two ice skaters standing motionless holding onto each. The first ice skater $($m$1=51$ kg$)$ pushes hard against the second ice skater $($m$2=71$ kg$).$ For the person sitting in the stands the second ice skater is now moving backwards at $3\mathrm{.}0$ m$/$s$.$ How fast is the first ice skater moving?